Hi @Joan Alcances ! You can bulk archive Cards that are not on a Board from the Card Manager. 

In the Card Manager, you can use the Filter by Attribute column on the right hand side of the page to filter for Board Count = 0:

Then, you can select all of the Cards and then bulk archive them: 

You can read more about bulk actions in the Card Manager here! Is this information helpful for you? 

This is really helpful for my collection clean up! Thank you, @Devon O'Dwyer